It is known that .alpha.-olefins obtained by thermal cracking of mineral oils, even after complex separation and refining treatments, contain small quantities of impurities, many of which, carbon monoxide included, have a detrimental effect on the Ziegler-Natta catalysts, particularly the high-yield catalysts, which generally contain a halogenated titanium halide compound supported on magnesium chloride, and an aluminum alkyl compound as the co-catalyst.
The carbon monoxide is generally present in the above mentioned .alpha.-olefins in a quantity ranging from 0.5 to 10 ppm (hereinafter understood to be ppm in moles), and at such levels it reduces considerably the efficiency of the Ziegler-Natta catalysts. On the other hand, it is not easy to further reduce the above mentioned concentrations of carbon monoxide in C.sub.2-4 .alpha.-olefins by simple distillation, given the low boiling point of said .alpha.-olefins. In particular, when the concentration of carbon monoxide is lower than or equal to 2 ppm, distillation becomes remarkably burdersome even from an economic standpoint.
The economic damage which derives from the presence of carbon monoxide in a polymerization process can be even worse when, as it often happens, the concentration of carbon monoxide fluctuates within the above mentioned range, thus causing a similar fluctuation of the polymer yield at the point where the latter exits the polymerization reactors, and this forces one to undertake continuous and costly operations to adjust the catalyst feeding systems, and not always with positive results. This proves how important it is to reduce the concentration of carbon monoxide in the .alpha.-olefins to levels below 0.03 ppm, where the influence on the behavior of the Ziegler-Natta catalysts is negligible.
The methods used up to date to reduce said carbon monoxide content in the .alpha.-olefins obtained by refining mineral oils (including thermal cracking), consist of using the capacity of some transition metal compounds, in particular copper compounds, in an aqueous solution form, or dispersed on inert solid supports (such as alumina or silica), to form complexes with the carbon monoxide.
For example, according to U.S. Pat. No. 3,014,973, the .alpha.-olefins to be purified, in particular ethylene and propylene, are absorbed in aqueous cuproammonium solutions, and subsequently selectively desorbed in such a way as to separate them from the carbon monoxide which remains complexed with the copper compound.
In order to further lower the carbon monoxide content, an additional treatment of the .alpha.-olefins by way of contacting them with the hydroxide of an alkali metal at temperatures higher than 170.degree. C. is expected.
From the data shown in U.S. Pat. No. 3,014,973, it is not obvious if by using the above mentioned process it is possible to reduce the concentration of carbon monoxide to less than 0.03 ppm. However, the above process is complex and costly, and requires an additional liquefying stage of the purified olefin, whenever, as it often happens in the case of propylene and 1-butene, one wants to carry out the polymerization in liquid monomer.